As the condition of a battery deteriorates, the equivalent internal resistance of the battery increases but its open circuit voltage does not similarly decrease. As such, the real condition of a battery can only be ascertained by placing the battery under load conditions prior to measuring its voltage. In this loaded condition, an operational current is drawn from the battery and an associated voltage drop occurs across the internal resistance of the battery which results in the measured voltage dropping below that of the open circuit battery. A significant drop in the voltage measured under load indicates that the battery is bad. Otherwise, the battery can be considered to still be good.
Batteries are often used for back-up power sources should the primary power source fail. In these instances, it would be inadvisable to continually draw an operation current from the battery as this would eventually deplete the battery and defeat its intended purpose of being available should the primary power source fail. Thus, it is typical to perform a battery test only at power-up and only over a short period of time which may be but a fraction of a second. Thus, the true condition of a battery is only really known for that short period of time during power-up. The battery is assumed to be good at other times if its measured voltage at that time was greater than a specified threshold value.